Calcium carbonate includes heavy calcium carbonate obtained by physically pulverizing natural white limestone and synthetic calcium carbonate obtained by chemical precipitation. The former heavy calcium carbonate has a wide particle size distribution and random shapes that are characteristic to physical pulverization as the production process wherein a natural product is pulverized, so that it is impossible to produce particles having a homogeneous particle diameter and uniform in shape.
On the other hand, the latter synthetic calcium carbonate is produced by chemical precipitation, it is possible to control the particle diameter and shape within a certain range. The calcium carbonate thus produced, having a characteristic shape such as spindle-shape, cubic shape, or columnar shape and having a narrow certain particle range, has specific functions and characteristic properties derived from the differences in shape and particle diameter which each calcium carbonate has and is used differently in the fields of paper making and various polymer materials depending on the functions and characteristic properties.
Among these synthetic calcium carbonates, spindle-shape calcium carbonate has a spindle-shape having a major axis of 1 to 4 μm and a minor axis of 0.2 to 1 μm (an average diameter on an electron microscope method; the same shall apply to the particle diameters shown below), exhibits a relatively high degree of whiteness and is excellent in economical efficiency, and hence it is used mainly as a filler for paper making in a large amount. On the other hand, not as a filler but as a pigment for use in coating on a surface, use is frequently made of cubic shape calcium carbonate having an average particle diameter of 0.1 to 0.3 μm or columnar calcium carbonate having a minor axis of 0.1 to 0.3 μm and a major axis of 0.5 to 2 μm, which exhibit a good dispersibility.
Recently, in paper making industry, for the purpose of reducing garbage generation and saving transportation cost, the tendency is toward saving weight of paper. The weight saving of paper has been attained by lowering weighing, thinning paper, or reducing an amount of a filler to be incorporated, but along with the weight saving, there arises a problem that opacity of paper decreases and hence a printed part is visible from the opposite side. As measures against the problem, many attempts at enhancing opacity have been made by incorporating various kinds of fillers or providing a special coating layer.
The substances for use as fillers for paper making or pigments to be incorporated into coating layers include kaolin, talc, silica, titanium dioxide, plastic pigments, or the like in addition to synthetic calcium carbonate and heavy calcium carbonate. These substances have different characteristic properties and are used solely or as mixtures depending on the quality required for the paper to be produced. Of these, especially titanium dioxide has the highest refractive index and hence is most effective for enhancing opacity of paper.
With regard to titanium dioxide, as a filler for paper making and a pigment for coating, particles having a particle diameter of 0.1 to 0.5 μm have been used but they have a property of easily flocculating owing to the influence of the pH of a paper stock and a coating agent, kind and concentration of a chemical agent for paper making to be added. In the case of filling or coating paper with titanium dioxide, the flocculation of titanium dioxide particles results in a heterogeneous distribution of titanium dioxide particles in paper and hence the opacity-enhancing effect inherent in titanium dioxide cannot be sufficiently realized.
Accordingly, when titanium dioxide particles are in a flocculated state, for attaining desired opacity, the amount of titanium dioxide should be necessarily increased, which results in not only disadvantage in cost but also retrogression against weight saving of paper. Especially, since titanium dioxide is expensive and has a large specific gravity as compared with other pigments and filler, it becomes important to achieve desired opacity by using a required minimum amount of titanium dioxide.
On the other hand, when titanium dioxide particles are in a monodispersed state, in the case of using the particles as a filler, fine titanium dioxide particles are difficult to entwine with pulp fibers and tend to drop out to waste water side in a paper making machine, so that yield ratio becomes low. Also, when the particles are included in a coating agent, the fineness of titanium dioxide particles results in increased viscosity of the coating agent, which causes deterioration of high-speed operability.
Therefore, when titanium dioxide is in a monodispersed state, there are also problems that the whole amount of titanium dioxide used cannot contribute to enhancement of opacity of paper owing to the low yield ratio as a filler and desired incorporating amount of titanium dioxide cannot be realized because of the increased viscosity of the coating agent. Under such a circumstance, many researches have been made aiming at enhancement of opacity of paper, wherein the opacity-enhancing effect inherent in titanium dioxide is efficiently utilized, and the following have been proposed.
For example, as pigments capable of enhancing opacity of paper suitably, there are proposed composite powder obtained by fixing a highly shielding pigment such as titanium dioxide onto the surface of an inorganic powder such as talc, calcium carbonate, clay, or kaolin using an organic binder as shown in JP-A-2-242998 and composite particles of silica and titania as shown in JP-A-9-286609.
In addition, JP-A-11-217797 discloses that titanium dioxide whose surface is treated with silica is excellent as a coating pigment for a ultra lightweight coat paper, and in Japanese Patent No. 2942182, a titanium dioxide pigment for paper making excellent in bonding property to pulp is obtained by treating a titanium dioxide surface with aluminum phosphate and alumina and/or magnesium oxide.
Moreover, as technologies of incorporating titanium dioxide into a coated layer of paper, there is proposed a technology of obtaining a printing paper having a light weight and a high opacity by coating a base paper containing needle-shape calcium carbonate having a specific particle diameter as a filler with a coating liquid containing rutile-type titanium dioxide having a specific oil absorbance and a specific particle diameter as in JP-A-7-331595 or a technology or the like of obtaining a coated paper for offset printing having an undercoat layer and a topcoat layer which is excellent in high-speed operability and has a high opacity by employing rutile-type titanium dioxide and calcium carbonate in a specific incorporating composition as a pigment for the topcoat layer as shown in JP-A-2000-54288.
As examples of filling titanium dioxide into paper as a filler, there have been proposed a process for producing a titanium dioxide-filled paper which is a base paper for decorative laminate used for melamine-decorative laminate, tap-decorative laminate, or the like excellent in opacity, wet strength, and yield ratio of titanium dioxide by lowering zeta-potential once with an organic electrolyte between after the addition of wet paper strength additive increasing agent and before the addition of alumina sulfate at the preparation of paper stock as shown in JP-A-8-246389, a filled paper excellent in the balance between optical properties such as opacity and degree of whiteness and physical properties, which is obtained by filling spindle-shape calcium carbonate, columnar calcium carbonate, and titanium dioxide in a predetermined mixing ratio as shown in JP-A-9-217292, and the like.
However, the above technologies of producing a filler and a pigment for paper making or paper which allow suitable enhancement of opacity require complex production processes and use of expensive starting materials, and hence it should be pointed out that industrial application thereof is problematic. Moreover, with regard to the technologies of incorporating titanium dioxide into paper, it is possible to realize opacity-enhancing effect inherent in titanium dioxide to some extent, but the opacity-enhancing function inherent in titanium dioxide is not thoroughly realized.
Furthermore, under the circumstance that further speeding up of a paper making process is required, the technologies are not satisfactory as technologies, which can meet the requirement, for obtaining coating agents incorporated with titanium dioxide. As mentioned above, titanium dioxide exhibits an effect on the enhancement of opacity as a pigment or filler for paper making, but the opacity-enhancing effect is realized at the most because of the flocculation of titanium dioxide particles and low yield ratio thereof. In addition, there is a problem that use of titanium dioxide may result in the increased viscosity of coating agent, which inhibits speeding up of a paper making process.